Light communication system employing superimposed currents applied to a high intensity light source



65-660 AU 233 EX XAMINER LIGHT COMMUNICATION SYSTEM EMPLOYING SUPERIIIPOSED CURRENTS APPLIED TO A HIGH INTENSITY LIGHT SOURCE Filed June 14, 1961 Nov. 10, 1964 E c. MUTSCHLER 3,156,826

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ATTORNEYS United States Patent 3,156,826 LIGHT COMMUNICATION SYSTEM EMPLOYING SUPERIMPOSED CURRENTS APPLIED TO A HIGH INTENSITY LIGHT SOURCE Edward C. Mutschler, Maplewood, N.J., assignor t0 Engelhard Hanovia, Inc., Newark, NJ., a corporation of New Jersey Filed June 14, 1961, Ser. No. 117,181 4 Claims. (Cl. 250--199) This invention relates to light communication systems and, more specifically, is concerned with arrangements for transmitting Morse Code signals by means of a light beam. Principal objects of the invention include increasing the speed of signal transmission and improving the signal-to-noise ratio.

Systems of the type under consideration are generally used under circumstances where the emitter and the receiver are not connected to each other by cables. Such circumstances prevail, for example, when it is intended to transmit signals between ships or between a ship and the shore. Heretofore, telegraphs using the Morse alphabet transmitted by means of a light beam carrier employed mechanical means for intermittently interrupting the light beam in accordance with the rhythm of the signals to be transmitted. Shutters or blinds, generally consisting of a plurality of parallel strips, performed the action of cutting off the light beam to produce the signals. This type of prior art devices is necessarily cumbersome and, in particular, a comparatively low speed of operation cannot be exceeded.

Accordingly, there is a need for replacing the mechanically operated shutter devices by systems in which the desired efiect is obtained by electrical means. Depending on the light source employed, difficulties of different types are encountered. Filament lamps, although they have been suggested for this purpose, emit a comparatively low light intensity and, additionally, since a definite time period is required until incandescence of the filament decreases or increases with changes of supply current, highspeed light transmission with filament lamps appears impossible for practical reasons.

Arc discharge lamps are generally used in light communication systems and preference is given to highpressure arc lamps. However, up to the present time, communication systems using arc discharge lamps for transmitting dash and dot signals are equipped with mechanical shutters. This was believed to be necessary since light emission must be interrupted between distinct signals, such as dashes and dots.

When replacing the shutter or blind systems by switches to shut off the lamp supply current, re-ignition between signals appeared a necessary requirement. Since re-igniting the are, once it is extinguished, is accompanied with considerable difiiculties and requires a certain definite period of time, attempts to replace the customary shutter systems by electrical switch means have either not been made or did not result in any practical design.

It has been found that, when supplying an arc discharge lamp selectively with two distinct types of current, each of them being applied to the lamp at an appropriate level, the lamp can be operated to emit signals composed of dashes and dots with a satisfactory speed and without the requirement to strike the arc again and again after each signal or series of signals. With respect to the mentioned levels of current, one of the two levels constitutes what is termed the quiescent level, generally used for the periods between signals, and the other level is that of the current applied to emit the signals in the desired rhythm to form dashes and dots. With respect to the quiescent level, it will be understood that it constitutes the minimum necessary for maintaining an arc in the lamp, or slightly above of prior art systems.

3,156,826 Patented Nov. 10, 1964 it. The level of actual signal current may by far exceed what is generally termed the rated power of the lamp, this term designating the average power which may be supplied to the lamp under continuous emission conditions without damaging it. It will be evident that, since the rated power is established for continuous operating conditions, the current level supplied to the lamp when emitting signals can be much higher, since the power is applied intermittently and not continuously, so that the average level is below that of the peaks actually applied.

As stated in the foregoing, the invention resides in the selection of two distinct types of current alternativelv applied to the lamp at one or the other level.

In accordance with the invention, a light communication system especially useful for transmitting signals composed of dashes and dots employing the Morse alphabet, includes an arc discharge lamp as the source of a carrier beam, and suitable circuitry which permits supplying selectively to the lamp either (1) pulsating current at a high level, suitably when emitting signals, or, (2) substantially steady current at a low level, which latter generally constitutes the quiescent level emitted between signals to maintain the are.

It was found that the speed of signals resulting from this type of emitter is limited only by the ability of the key operator, since the lamp emission practically immediately shifts from high intensity to low intensity. No shutter or any other mechanical means for interrupting the light beam are needed, since switching from substantially steady low-level current to high-level pulsating current may be conveniently effected by using a conventional telegraph key. Considerable progress is achieved, as compared to manually operated systems, inasmuch as the rate of transmission is increased to twice or three times that A further advantage resides in the fact that the system is controlled by switching alternating current exclusively. As a result, the well known shortcomings of switching heavy direct current, residing in the formation of sparks at the switch contacts, are avoided.

A further and more important advantage of the present invention is due to the fact that the pulsating current produced signals can be filtered on the receiving end of a complete system, for example by means of a band pass circuit, so that the signal-to-noise ratio is significantly improved.

In accordance with one of the more essential features of the invention, the two types of different current levels are provided by arrangements to supply the arc discharge lamp selectively, or alternately, with high-level unfiltered, full-wave rectified alternating current or low-level, filtered, full-wave rectified alternating current.

The invention includes, by Way of example, one embodiment using the principle set forth in the foregoing by means of circuitry which applies full-wave rectified alternating current, suitable for emitting signals, and additional circuitry for intermittent filtering and limiting the lamp supply, which latter condition results in the emission of light at the low-level which is the quiescent level.

In accordance with another feature of the invention, as exemplified by another embodiment, low-level steady current is continuously supplied to the lamp for maintaining the arc and the light emission at a minimum, and the signals are produced by intermittently superimposing highlevel pulsating current on the steady low-level supply.

A still further feature of the present invention includes appropriate circuitry for continuously providing current supply at an intermediate level, so that the lamp may be used as a search light. It will be understood that such intermediate level may be equal but should not substantially exceed the average power rate of thelamp in order to preserve the life thereof.

In practice, the lamp is suitably simmered down to a quiescent level not greater than of 1ts rated POWer level, and is pulsed to above 200% of it. This res in a light intensity change of approximately 100 to 200 times. The single-frequency pulse nature of the light signals permits electrical filtering of therec l Signal with greatly improved signal-to-noise ratio.

The invention will be further illustrated and Wlll be more clearly understood by consideration of the accomn in drawin in which: F lGRE l if a diagram schematically illustrating the lamp current, and therewith the emitted light intensity, in a system according to the invention;

FIGURE 2 is a schematic circuit diagram illustrating one embodiment of the invention;

FIGURE 3 is a schematic block diagram illustrating a receiver suitable for cooperation with an emitter of the invention; and

FIGURE 4 is a schematic circuit diagram illustrating another embodiment of a light communication system which permits the use of the lamp as a search light.

In the diagram of FIGURE 1, the lamp current, and therefore the intensity of emitted light, is plotted along the time axis to illustrate one of the underlying principles of the invention. Although, theoretically speaking, pulsating currents of any type may be used, it has been found preferable to employ that shown in FIGURE 1 which is obtained as the output of a full-wave rectifier fed with alternating current. The frequencies of the alternating current supply may be 400 cycles per second. The resulting pulsating direct current illustrated in FIG- URE 1 then has a frequency of 800 cycles per second.

For obvious reasons, the pulsating current is used for emitting the signals, while the lamp is simmered down to the quiescent level during the periods between signals.

In addition to the plot illustrating the lamp current or light intensity and the quiescent level, the horizontal dashed line in FIGURE 1 designates the rated power level which, when interpreted in watts, represents the average power which may safely be applied to the lamp. Since signals are emitted intermittently, the peak value of lamp current may exceed the rated power level and, in practice, it has been found that peak values of more than twice the rated power level are permissible.

The circuitry shown in FIGURE 2 illustrates one embodiment of the emitter in a light communication system performing in the manner shown in FIGURE 1. Generally speaking, the arrangement of FIGURE 2 includes circuit elements for supplying full-wave rectified alternating current to the lamp, either with a filter inserted in the circuitry for simmering the lamp output down to the quiescent level, or without the filter, whereby the full unfiltered output is applied to the lamp.

More specifically, the circuit of FIGURE 2 includes input terminals 10 by which 110 volts alternating current, sultably of 400 cycles per second, may be applied to a transformer 12. The transformer output is rectified by means of the arrangement designated by reference numeral 14, suitably a rectifier assembly in bridge connectron. The rectified output is applied to an arc discharge p 16 Over a signalling switch, suitably a conventional telggraph key 18.

or the Sake of completeness, the circuit show FIGURE 2 includes a radio frequency ignition ci r ctiii signated by the block 20 which is supplied with alternating current over terminals 22, and of which the output may be applied to the lamp 16 through transformer 24. Ignition circuits are well known in the art and therefore need not be described in detail. In order to prevent the radio frequency currents from damaging the rectifier 14, the circuit includes a filter comprising a pair of chokes 26 and 28, and capacitors 30 and 32, the capacitor 32 performing as a radio frequency by-pass capacitor.

In accordance with the embodiment of the invention illustrated in FIGURE 2, means are provided for selectively and intermittently supply to the pf of both, the unfiltered full output from rectifier bridge 14 or the same output, but with a filter inserted in the lamp supply lines 34 and 36. The filter includes capacitor 38 and resistor 40. With the switch or key 18 in its normally open position shown in FIGURE 2, the switch arm 41 cooperates with the upper contact 42. When in this position, the filter consisting of capacitor 38 and resistor 40, the latter performing as a ballast, are inserted into the lamp supply line 36, with the ballast 40 in series connection. Then, low-level full-wave rectified alternating current corresponding to the quiescent level is supplied to the lamp. Assuming that the rated power of the lamp is 300 Watts, the quiescent level may be about 30 watts, with resistor 40 having a resistance of about 40 ohms. Suitably, a variable resistor may be used so that the quiescent level may be adjusted in accordance with the type of lamp employed.

When the key 18 is pressed down so that the arm 41 contacts the lower contact point 44, signals are emitted because the circuit elements 38 and 40 forming the filter are removed from the current supply and, consequently, the full-wave rectified unfiltered output from the bridge circuit 14 is fed to the lamp 16. As stated and for the reasons given above, the peak value of the resulting pulsating current of which the lowest frequency is 800 cycles per second with some higher frequency components of very low amplitude in the waveform, may exceed the rated power level as shown in FIGURE 1.

The very low current supplied to constitute the quiescent level is selected in such a manner that an arc is maintained in the lamp which constitutes the minimum requirement under these conditions. On the other hand, and for obvious reasons, the quiescent level is maintained as low as possible, in order to achieve an optimum signal-tonoise ratio. The filtering capacitor 38 of for example 500 microfarads, and resistor 40 are suitably dimensioned to provide no more than 5% ripple associated with quiescent level emission.

A receiver suitable for use in conjunction with the emitter system described above is shown in a schematic diagram in FIGURE 3. In this regard, it should be noted that the comparatively slow operating communication systems of the prior art using shutters were frequently operated by visual pick-up in view of the low speed of transmission. The system of the present invention permitting high speed signalling is suitable for reception of any type, such as audio pick-up, recording, or both. FIGURE 3 illustrates a receiver circuit which may be used. A photocell 46 picks up the signals and is included in a sensing circuit comprising a current source 48 and resistor 50. The voltage across resistor 50 is applied to the audio output circuit 52 through an 800 cycle band pass circuit 54.

At this point, it should be noted that the frequencies of the power input in FIGURE 2 was assumed to be 400 cycles per second. Accordingly, the lowest frequency component of pulsating light signals is 800 cycles per second, as indicated in FIGURE 1. As a result of the presence of the band pass circuit 54 in the receiver of FIGURE 3, the direct current components are eliminated, and this fact is considered a specific advantage resulting from the use of unfiltered pulsating signal current and filtered low-level and low-ripple filtered quiescent level current. It will be understood that the filtering circuit 54 eliminates direct current components and therefore considerable increase of the signal-to-noise ratio is achieved. The receiver illustrated in FIGURE 3 may include a loud speaker 56 for audio reception and rectifying circuitry 58, as well as conventional recording equipment 60.

The system described in connection with the circuitry of FIGURE 2 performs in such a manner that either unfiltered or filtered full-wave rectified alternating current is selectively, or intermittently, supplied to the lamp to emit light signals depending on the position of the signalling key 18, as described above. The system illustrated by the circuitry of FIGURE 4, generally speaking, performs superimposition instead of substitution. Accordingly, the lamp is continuously fed with full-wave rectified, filtered, low-level, low-ripple alternating current to provide the quiescent level. For producing signals, a pulsating current which is an unfiltered fullwave rectified alternating current is superimposed on the continuously supplied quiescent level current. As described in more detail below, the system of FIGURE 4 includes a current limiting resistor which permits using the lamp at a constant level for illuminating purposes, for example as a search light.

The circuitry shown in FIGURE 4 includes input components identical with those shown in FIGURE 2 which are terminals 62, and an input transformer 64. With the switch 66 closed, the transformer output is rectified by means of bridge circuit 68 and applied to the lamp 70 over the filter 72 which includes a choke 74 and capacitors 76 and 78. Furthermore, a current limiting resistor 80 is provided in the lamp supply line. The circuitry so described supplies full-wave rectified filtered low-level current to the lamp 70 for maintaining the are at the quiescent level. Accordingly, switch 66 will be maintained closed whenever the system is under operating conditions.

For the sake of completeness, an ignition circuit for starting the lamp is shown in FIGURE 4. This ignition circuit receives power from the output of transformer 64 over lines 82 and 84 and includes a transformer 86, spark gap 88, capacitor 90 and radio frequency transformer 92. The ignition circuit is energized by closing switch 94 in supply line 84 and operates in a manner well known in the art, as stated above. A radio frequency filter 96 is provided for the reasons given in connection with FIG- URE 2.

As mentioned in the foregoing, in order to emit signals, a pulsating current is superimposed on the quiescent level current supplied to the lamp by closing switch 66. The signalling current is drawn from supply lines 82 and 84 and supplied to a transformer 98 through telegraph key 100. As a result of this arrangement, the key 100 handles alternating current so that damaging the key contacts by arcing is avoided. The output from transformer 98 is full-wave rectified by bridge circuit 102 and, with switch 104 closed, the unfiltered full-wave rectified output from transformer 98 appears at the connecting points 106 and 108. It can be seen that the signalling current derived from the output of rectifier 102 is supplied to the lamp as a current which, since applied in parallel connection, is superimposed on the output current supplied from rectifier 68 through filter 72. Consequently, the light output from the lamp 70 corresponds to that illustrated in FIG- URE 1, the result being obtained by superimposition of both types of current whereas the circuitry of FIGURE 2 operates by substitution to obtain the same result.

It has been mentioned that the system illustrated in FIGURE 4 can be used in such a manner that the lamp 70 operates as a search light with continuous emission. For this purpose, the telegraph key 100 may be provided with means for blocking it in the closed position. Alternatively a switch 110 may be provided for shortcircuiting the key 100. It has been assumed that with switch 104 closed, the power supply to the lamp when operating the telegraph key 100 reaches the peak value shown in FIGURE 1, which is above and may be higher than twice the rated power level. In order to reduce the current for continuously operating the lamp 70 as a search light, current limiting resistor 112 is provided in the output line of rectifier 102. It will be understood that with switch 104 short-circuiting resistor 112 in its closed position, switch 104 is maintained open when using the lamp 70 as a search light. Under these circumstances, resistor 112 reduces the current supplied through switch 110, transformer 98 and rectifier 102 to the lamp, so that the power actually supplied to the lamp, and in addition to the current applied to it through rectifier 68, does not exceed the rated power level. As a result, the lamp operates safely as a search light with switch 110 closed and switch 104 open. When it is desired to emit telegraph signals, switch 110 is maintained open and switch 104 remains closed so that operation of key results in applying high-level signalling current to the lamp, as described above.

In the foregoing, resistor 112 and switch 104 have been shown as arranged in the circuitry of FIGURE 4. It will easily be understood that a similar modification is possible with respect to the circuitry of FIGURE 2 by inserting a current limiting resistor in either of the lines 34 and 36 for using lamp 16 as a search light. In accordance with such modification, the key 18 will be provided with means for blocking it in a closed position to continuously cooperate with contact point 44, or a shortcircuiting switch similar to switch may be provided.

Under certain circumstances, when it is desired to emit secret signals not easily detected, a filter indicated at 114 in FIGURE 4 may be used for transmitting infrared light signals, with the visible frequency range eliminated from the lamp output.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. A light communication system comprising an arc discharge lamp having a predetermined rated power, a source of alternating current; a first circuit for supplying the lamp with pulsating current at a higher level than the rated power of the lamp, said first circuit including the source of current connected to the lamp through a full wave rectifier; and a second circuit for supplying current to the lamp at a lower level than the rated power of the lamp but sufiicient to maintain an arc and including the source of current connected to the lamp through a full wave rectifier, and having connected in the circuit between the rectifier and the lamp a resistor in series with the lamp and filter means to eliminate pulses in the current.

2. A light communication system as set forth in claim 1 including signal switch means connected to open a1- ternatively the first circuit while closing the second and vice versa.

3. A light communication system as set forth in claim 1 including a signal switch connected in series in the first circuit between the source of current and the rectifier.

4. A light communication system as set forth in claim 1 including a resistor connected in series between the rectifier and the lamp in the first circuit and a switch connected to short circuit the resistor, said resistor having a resistance value sufficient to reduce the total power applied to the lamp to approximately the rated power of the lamp.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A LIGHT COMMUNICATION SYSTEM COMPRISING AN ARC DISCHARGE LAMP HAVING A PREDETERMINED RATED POWER, A SOURCE OF ALTERNATING CURRENT; A FIRST CIRCUIT FOR SUPPLYING THE LAMP WITH PULSATING CURRENT AT A HIGHER LEVEL THAN THE RATED POWER OF THE LAMP, SAID FIRST CIRCUIT INCLUDING THE SOURCE OF CURRENT CONNECTED TO THE LAMP THROUGH A FULL WAVE RECTIFIER; AND A SECOND CIRCUIT FOR SUPPLYING CURRENT TO THE LAMP AT A LOWER LEVEL THAN THE RATED POWER OF THE LAMP BUT SUFFICIENT TO MAINTAIN AN ARC AND INCLUDING THE SOURCE OF CURRENT CONNECTED TO THE LAMP THROUGH A FULL WAVE RECTIFIER, AND HAVING CONNECTED IN THE CIRCUIT BETWEEN THE RECTIFIER AND THE LAMP A RESISTOR IN SERIES WITH 